৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Zeolite process (Ion-exchange method): In this method, zeolite (NaAlSiO₄·H₂O) is used, which replaces calcium (Ca²⁺) and magnesium (Mg²⁺) ions in hard water with sodium (Na⁺) ions, thus softening the water.

Coagulation process: Used to remove suspended impurities, not for softening water.

Lime-soda process: Removes calcium and magnesium by converting them into insoluble precipitates.

Electrodialysis: Removes ions using an electric field, usually applied in desalination.

 Therefore, the method of water softening that involves replacing calcium and magnesium ions with sodium ions is the Zeolite process

In the lime-soda process for water softening:
Lime (Ca(OH)₂) removes temporary hardness (due to bicarbonates) by precipitating CaCO₃ and Mg(OH)₂.
Soda ash (Na₂CO₃) is added to remove non-carbonate hardness (caused by chlorides, sulfates, and nitrates of Ca²⁺ and Mg²⁺).
Reaction:
   CaCl₂+Na₂CO₃⟶CaCO₃↓+2NaCl
Here, Ca²⁺ (from CaCl₂, CaSO₄, etc.) reacts with carbonate ions to form insoluble CaCO₃.

This reduces hardness that lime alone cannot remove.

Reverse Osmosis (RO) is a water purification method where pressure greater than the osmotic pressure is applied to force water through a semipermeable membrane, leaving dissolved salts and other impurities behind.
The membrane allows water molecules to pass but rejects most dissolved salts, organic matter, and microorganisms.
​
​Other options:
​(ক) RO increases the salinity of water → Completely wrong; it reduces salinity.
(খ) RO requires low pressure and low osmotic gradient → Incorrect; RO requires high pressure to overcome natural osmotic pressure.
(গ) RO is effective only for removing suspended solids, not dissolved salts → False; its main advantage is removing dissolved salts, unlike normal filtration.

Sodium hexametaphosphate (SHMP) acts as a sequestering agent^** in boiler water conditioning.

It combines with Ca²⁺ and Mg²⁺ ions (which cause hardness) to form soluble complexes, preventing the precipitation of CaCO₃, Mg(OH)₂, and other salts that cause scale formation.

This method is called phosphate conditioning, widely used for high-pressure boilers.
​
​** In chemistry, a sequestering agent is an organic compound capable of linking metal ions or molecules together to form complex ring-like structures known as chelates. Although similar, a sequestering agent is different than a chelating agent

Activated carbon filtration is highly effective in:
 1. Removing taste and odor (adsorbs organic compounds)
 2. Removing residual chlorine (through catalytic reduction)
 3.  Adsorbing organic impurities (pesticides, VOCs, etc.)
However, it does not remove hardness (Ca²⁺ and Mg²⁺ ions), because these are dissolved inorganic salts, which require processes like ion exchange, lime-soda softening, or RO.
​
​Other Options:
​(ক) Taste and odor → Removed effectively by adsorption.
(খ) Residual chlorine → Neutralized by catalytic action on carbon surface.
(ঘ) Organic impurities → Adsorbed strongly.

In sedimentation (clarification), efficiency of suspended solids removal primarily depends on the surface overflow rate (SOR), also called surface loading rate, which is:
SOR=Flow rate/Surface area of tank
​
According to Hazens’ theory, settling efficiency is controlled by the surface area, not the depth, because particles settle at a constant velocity, and any particle with a settling velocity greater than the overflow rate will settle out.

Coagulation is the process of neutralizing the electrical charges on colloidal particles (such as clay, silt, organic matter) in water so they can come together (flocculate) and settle out.
Colloids are negatively charged, so they repel each other and do not settle naturally.
When alum is added to water:
    1. It dissolves and hydrolyzes to form Al³⁺ ions.
    2. These ions neutralize the negative charges on colloids.
    3. Then, aluminum hydroxide [Al(OH)₃] precipitate forms, creating a gel-like floc that traps suspended and colloidal impurities.

Reactions involved:
Al₂(SO₄)₃+6H₂O⟶2Al(OH)₃↓+3H₂SO4

The floc formed settles down in the sedimentation tank.
Alum works best in slightly alkaline water (pH 6–8) because Al(OH)₃ formation is favored.

Wastewater disinfection methods can be broadly categorized as:
  1. Physical → Use physical agents like heat or radiation.
  2. Chemical → Use chemicals like chlorine, ozone.
Chlorination is a chemical method (it uses chlorine or chlorine compounds to kill pathogens), not a physical method.
Common physical disinfection methods:
  1. UV radiation → Damages microbial DNA, preventing reproduction.
  2. Gamma radiation → Ionizes cellular components, killing microbes.
  3. Boiling → Heat kills pathogens effectively.

Activated carbon is a highly porous material with an enormous surface area (up to 1000 m²/g).
It removes contaminants by physical adsorption, which involves Van der Waals forces and sometimes minor chemisorption.
This makes it very effective for removing:
   Organic compounds (taste, odor, color)
    Volatile organic compounds (VOCs)
    Chlorine and some disinfection by-products
Its primary mechanism is not chemical reaction, but adsorption on a porous surface.
​
​(ক) Ion exchange → Performed by ion-exchange resins, not by activated carbon.
(খ) Chemical oxidation → Requires oxidizing agents like chlorine, ozone.
(ঘ) Electrochemical precipitation → Used in electrochemical treatment, not in activated carbon filtration.

Chlorination efficiency depends on:
     Turbidity: Lower turbidity improves efficiency because suspended solids shield microorganisms from chlorine.
      pH: Neutral pH (around 7) is ideal because HOCl (hypochlorous acid)—the most effective disinfecting species—is predominant at pH 6–7.
             At high pH (>8), HOCl converts to OCl⁻, which is less effective.
Contact time and temperature: Higher temperature generally helps, but if organic load is high, it consumes chlorine, reducing effectiveness.
The presence of ammonia forms chloramines, which are weaker disinfectants than free chlorine.
​
​(খ) Presence of ammonia → Leads to chloramine formation, reducing immediate disinfection efficiency.
(গ) High pH (>10) → Converts HOCl to OCl⁻, making chlorination less effective.
(ঘ) High temperature combined with high organic load → High organic matter consumes chlorine, leaving less free chlorine for disinfection.

Advanced Oxidation Processes (AOPs) are chemical water treatment methods designed to degrade persistent organic pollutants.
The key mechanism is the generation of highly reactive radicals, mainly the hydroxyl radical (-OH).
Hydroxyl radicals have:
  1. Extremely high oxidation potential (~2.8 V, higher than ozone or chlorine)
  2. Non-selective reactivity, meaning they can react with a wide range of organic compounds, breaking them down into CO₂, water, and inorganic salts.

​Common AOPs include:
O₃/H₂O₂
UV/H₂O₂
Fenton process (Fe²⁺/H₂O₂)

In a municipal water supply system, service reservoirs are often called the “lungs of the city” because they store treated water and help regulate pressure and flow throughout the distribution network.
They serve several key purposes:
       1. Balancing supply and demand – they provide water during peak demand periods.
       2 . Pressure regulation – maintaining consistent pressure in the mains.
       3. Emergency storage – supplying water during outages or maintenance.
Analogy to lungs: Just as lungs store and release oxygen to maintain a steady supply for the body, service reservoirs store and release water to maintain a steady supply to the city.
​
​(খ) Distribution mains → These act like arteries, transporting water, not storing it.
(গ) Water treatment plant → Cleans water, like a filter, but doesn’t provide storage or buffering.
(ঘ) Intake structure → Only draws raw water from the source; does not regulate supply.

“Red water” refers to reddish-brown water in the distribution system, usually caused by oxidized iron (Fe³⁺) or manganese (Mn⁴⁺) particles.
If iron or manganese concentrations in treated water are too high, they can precipitate in pipes, leading to:
   1. Staining of clothes and plumbing fixtures
   2. Bad taste and color complaints
   3. Potential microbial growth on pipe scales
Control before distribution:
Remove Fe and Mn through oxidation followed by filtration or water softening processes.
Maintaining low levels ensures water remains clear and aesthetically acceptable.

Break-pressure tanks (BPTs) are intermediate storage tanks installed along long water transmission pipelines, especially in hilly or sloping terrains.
Purpose:
   When water flows down a slope, pressure increases due to gravity (hydrostatic head).
   Excessive pressure can damage pipes, joints, and valves, causing leaks or bursts.
   A break-pressure tank intercepts the flow, reduces the pressure, and then allows controlled flow downstream.

​They may also provide a small buffer storage, but the primary reason is pressure control.
​
​(ক) To store water for emergencies → Not the main purpose; reservoirs are used for emergency storage.
(খ) To prevent freezing in cold climates → Not applicable; freezing is managed by insulation or buried pipelines.
(ঘ) To increase water velocity → BPTs reduce, not increase, hydraulic pressure and velocity.

Chlorine demand is the amount of chlorine consumed by reactions with impurities in water (organics, ammonia, iron, etc.).
Chlorine residual is the amount left in water after these reactions, which ensures disinfection.
Total chlorine to be applied:
    Chlorine applied=Chlorine demand+Desired residual

Given:
Chlorine demand = 2 mg/L
Desired residual = 0.5 mg/L
                 
​Chlorine applied = 2+0.5 = 2.5 mg/L.

The grid iron system is a network layout where pipes are laid in a rectangular or square grid, resembling a spider’s web.
Advantages:
 Provides multiple pathways for water to reach consumers.
 Improves reliability—if one pipe fails, water can be rerouted.
 Maintains adequate pressure and flow across the network.

​Comparison with other systems:
Ring system (ক) → Loops around the city; mainly for long continuous supply along main roads.
Radial system (খ) → Pipes radiate from a central reservoir; may have pressure variation issues at the edges.
Dead-end system (ঘ) → Pipes terminate at endpoints; less reliable, prone to stagnation.

Dissolved oxygen (DO) in boiler feed water is highly reactive with metal surfaces, especially iron and steel.
High DO leads to:
         Localized corrosion or pitting in boiler tubes and pipes.
        Pitting is dangerous because it creates small holes, which can cause leaks and failures under high pressure.

Prevention:
      Use deaerators to remove oxygen.
      Add oxygen scavengers like hydrazine (N₂H₄) or sodium sulfite (Na₂SO₃).
​
​ক) Scale formation → Caused by hardness (Ca²⁺, Mg²⁺), not oxygen.
(খ) Priming and foaming → Caused by suspended solids, high alkalinity, or sudden pressure changes, not DO.
(গ) Caustic embrittlement → Caused by high concentration of caustic (NaOH) in boiler, not oxygen.

Phosphate treatment is an internal boiler water conditioning method used to prevent scale formation.

​Mechanism:
Hardness in feed water (Ca²⁺, Mg²⁺) can form insoluble scale like CaCO₃ on boiler surfaces.
Sodium phosphate (Na₃PO₄) reacts with Ca²⁺ and Mg²⁺ to form soft, non-adherent sludge:
 3Ca²⁺+2PO₄³⁻⟶Ca₃(PO₄)₂↓
This sludge can be blown down from the boiler, preventing hard scale deposition.

​Advantages:
      Protects boiler tubes from scaling and overheating.
     Compatible with high-pressure boilers.

High-pressure boilers are highly sensitive to oxygen corrosion, which can cause pitting and tube failure.
Sodium sulfite (Na₂SO₃) is commonly added to boiler feed water as an oxygen scavenger:

2Na₂SO₃+ O₂⟶ 2Na₂SO₄
 
​Benefits:
     Converts dissolved oxygen into a harmless compound (sodium sulfate).
     Protects boiler tubes and pipes from localized corrosion.
​
​(ক) Preventing foaming → Done by anti-foaming agents, not sodium sulfite.
(খ) Removing hardness → Done by lime, soda, or phosphate treatment, not oxygen scavengers.
(ঘ) Reducing silica content → Silica removal requires deionization or reverse osmosis, not sodium sulfite.

High-pressure boilers are more susceptible to corrosion, particularly oxygen corrosion and caustic attack.
 
​Maintaining an alkaline pH in feed water (typically 10.5–12.0) helps to:
  Prevent acidic corrosion caused by dissolved CO₂ or oxygen.
  Reduce metal leaching from boiler tubes.
  Ensure effective operation of phosphate and oxygen scavenger treatments.

​Methods to maintain pH:
Addition of sodium hydroxide (NaOH) or sodium phosphate.
​
​(ক) 4.5 – 6.0 → Acidic range; would increase corrosion risk.
(খ) 6.5 – 7.5 → Neutral range; not sufficient for high-pressure boilers.
(গ) 8.5 – 9.5 → Mildly alkaline; better than neutral, but still lower than recommended for high-pressure systems.

Silica (SiO₂) is highly soluble in boiler water at high temperatures and pressures.
When water steam-flashes into high-pressure steam, silica precipitates out, especially in turbine blades and steam lines, forming a hard, glassy deposit.

​Consequences:
Reduced turbine efficiency
Imbalance and vibration
Increased maintenance costs

​Control measures:
     Use deionization, reverse osmosis, or silica removal softeners for feed water.
    Limit silica concentration according to boiler pressure (e.g., <10 ppb for supercritical turbines).

In rural Bangladesh, the primary concern in groundwater is the high concentration of iron and arsenic in tube wells.
 
​These contaminants pose severe health risks:
     Iron: causes taste, staining, and clogging problems.
    Arsenic: chronic exposure leads to skin lesions, cancers, and organ damage.

Water treatment technology is therefore selected based on:
     Removal of arsenic (e.g., coagulation-filtration, adsorption, or adsorption media)
     Iron removal (e.g., oxidation followed by filtration)

Slow sand filtration (SSF) is a biologically active filtration process widely used in rural and municipal water supply in Bangladesh.

Reasons for continued preference despite advanced technologies:
Low operational cost: Does not require chemicals or high energy input.
Simple technology: Can be operated and maintained by minimally trained personnel.
Effective pathogen removal: Through biological action and physical straining.
Sustainability: Suitable for small and medium-sized communities.

Limitations:
     Low filtration rate (slow flow)
     Less effective for removing dissolved chemicals like arsenic or fluoride (requires additional treatment)

Monsoon impact on surface water:
During the monsoon season in Bangladesh, heavy rainfall and flooding cause rivers, canals, and ponds to carry a large load of suspended solids, including silt, clay, and organic debris.
This leads to high turbidity and sometimes coloration in raw water, which can dramatically exceed normal treatment capacities.

Challenges for water treatment plants:
1. Coagulation and sedimentation:
   High turbidity requires higher doses of coagulants (like alum or ferric salts) to form flocs that settle efficiently.

2. ​Filtration:
   Slow sand or rapid sand filters face faster clogging, requiring more frequent backwashing, which increases operational workload.

3. Disinfection efficiency:
   Suspended solids can shield microorganisms from disinfectants like chlorine, reducing pathogen removal efficiency.

4. Operational stress:
   Treatment plants may need to adjust flow rates and chemical dosages frequently to maintain safe water quality.

Electrodialysis (ED) is a membrane-based desalination process used to remove salts from water by applying an electric potential difference across the solution. The process uses alternating cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs) arranged between electrodes.

1. Cation-exchange membranes (CEMs):
Contain fixed negative charges (usually sulfonic acid groups).
Attract and allow positively charged ions (cations like Na⁺, K⁺, Ca²⁺) to pass through, while repelling anions.

2. Anion-exchange membranes (AEMs):
Contain fixed positive charges (usually quaternary ammonium groups).
Attract and allow negatively charged ions (anions like Cl⁻, SO₄²⁻) to pass through, while repelling cations.

Mechanism of selectivity:
When an electric potential is applied, cations move toward the cathode, anions toward the anode.
Only ions of the opposite charge to the fixed groups on the membrane can pass through.
This charge-based selectivity ensures that ions migrate in the desired direction, leaving desalinated water in the diluate compartments.

Ultrafiltration (UF) membranes are widely used for removing suspended solids, colloids, bacteria, and macromolecules from water.
Long-term performance limitations are mainly due to:
​
1. Concentration polarization:
Solutes or particles accumulate near the membrane surface because water passes through the membrane faster than solutes.
This creates a higher local solute concentration, reducing permeate flux.

2. Fouling:
Deposition of organic matter, biofilms, colloids, or inorganic salts on the membrane surface or within pores.
Fouling leads to:
      Reduced permeate flow rate
      Increased transmembrane pressure (TMP)
     Shortened membrane lifespan

Mitigation strategies:
Periodic backwashing and chemical cleaning
Pretreatment of feed water (coagulation, sedimentation)
Optimizing crossflow velocity to reduce concentration polarization

Activated carbon is used in water treatment to remove organic micropollutants, taste, odor, and color.

Powdered Activated Carbon (PAC):
    1. Particle size: Very fine powder (typically < 100 µm)
    2. Surface area exposure: Large external surface area relative to particle volume
    3. Kinetics: Adsorption occurs faster because pollutants can quickly access adsorption sites.

Granular Activated Carbon (GAC):
    1. Larger particle size (0.2–5 mm)
    2. Adsorption is slower, mainly used in fixed-bed continuous systems.

Application:
PAC is added directly to water in batch processes or in coagulation tanks for rapid removal of trace organics.
After treatment, PAC is removed by sedimentation or filtration.

Hybrid systems combining ultrafiltration (UF) and activated carbon (AC) are used to achieve high-quality water by removing both particulates and dissolved organics.

Why UF is placed first:
1. Suspended solids and colloids removal: UF membranes effectively remove turbidity, bacteria, and colloidal particles.
2. Protects the activated carbon:
      Prevents rapid fouling or clogging of the AC bed.
      Extends the adsorption efficiency and lifespan of activated carbon.
3. Improved water clarity: Ensures that the AC bed primarily deals with dissolved organics, which are its main target.

Sequence benefit: UF → AC ensures stable operation, lower maintenance, and optimal contaminant removal.

BOD (Biochemical Oxygen Demand):
Measures the amount of dissolved oxygen required by microorganisms to biologically oxidize the organic matter in water over a period (commonly 5 days at 20°C, called BOD₅).
Only accounts for biodegradable organic matter.

COD (Chemical Oxygen Demand):
Measures the total oxygen equivalent of all organics in water that can be chemically oxidized using strong oxidants (like potassium dichromate in acidic conditions).
Includes both biodegradable and non-biodegradable organics.
Gives faster results (a few hours) compared to the 5-day BOD test.

BOD (Biochemical Oxygen Demand) measures the oxygen required by microorganisms to oxidize biodegradable organics over 5 days.

​COD (Chemical Oxygen Demand) measures the oxygen required to chemically oxidize all oxidizable organics, including both biodegradable and non-biodegradable compounds.

​Interpretation of BOD ≪ COD:
A large gap between COD and BOD indicates that a significant portion of the organic matter in wastewater is non-biodegradable.
Microorganisms cannot break down these compounds easily, so BOD is low, while COD still reflects total oxidizable content.

​Implications:
Treatment may require advanced chemical or physicochemical methods instead of relying solely on biological treatment.

COD is determined by oxidizing organic matter chemically using a strong oxidizing agent.

The most widely used method is the open reflux method, where potassium dichromate (K₂Cr₂O₇) in acidic medium (with sulfuric acid and silver sulfate as a catalyst) oxidizes the organic matter.

The amount of dichromate consumed is measured by titration with ferrous ammonium sulfate (FAS), which indicates the oxygen equivalent of the organic matter.

The lime-soda process is a chemical water softening method.
Soda ash (Na₂CO₃) is added to react with soluble calcium salts (like CaSO₄ or Ca(HCO₃)₂) to form insoluble calcium carbonate (CaCO₃), which can then be removed by sedimentation or filtration:
Ca²⁺+Na₂CO₃→CaCO₃↓+2Na⁺
Lime (Ca(OH)₂) is used mainly to remove magnesium as magnesium hydroxide (Mg(OH)₂).

Other options:
(ক)Remove dissolved oxygen: Not the purpose of soda ash.
​(গ)Precipitate magnesium: Done by lime, not soda ash.
(ঘ)Disinfect water: Requires chlorine or similar disinfectants, not soda ash.

Sodium hexametaphosphate (SHMP) is a sequestering agent used in boiler water treatment.
Function:
 1,It reacts with calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the water to form soluble complexes.
 2,This prevents the formation of scale deposits on boiler surfaces, which can reduce heat transfer efficiency and cause corrosion.

Other options:
​(ক)Increase alkalinity: Achieved by adding sodium carbonate or hydroxide, not SHMP.
(খ)Reduce dissolved oxygen : Done by de-aerators or oxygen scavengers like sodium sulfite.
(ঘ)Control pH : pH adjustments are done separately using alkalis or acids.

High dissolved oxygen in boiler feed water is highly corrosive to metals (iron, steel) because oxygen reacts with iron to form rust (Fe₂O₃·xH₂O):
4Fe+3O₂+6H₂O→4Fe(OH)₃ (which dehydrates to rust) 
This type of corrosion is called oxygen pitting, which creates deep pits and weakens the boiler walls.

Other options:
(ক) Priming : Caused by sudden boiling and entrainment of water droplets with steam, not oxygen.
(খ) Foaming: Due to oils, alkalinity, or dissolved solids—not oxygen.
​(গ) Hard scale formation : Caused by hardness salts (Ca²⁺, Mg²⁺), not oxygen.

Activated carbon filtration works mainly through adsorption and is excellent at improving:
1.Color (removes organic matter causing discoloration)
2.Taste (removes chlorine and organics)
3.Odor (removes smell from organics, chlorine, sulfur compounds)

However, it does not remove hardness ions like Ca²⁺ and Mg²⁺ because these are dissolved inorganic salts, which require ion exchange (softening) or reverse osmosis to be removed.

1.The efficiency of sedimentation tanks is governed by Stokes' Law, which relates settling velocity to particle size, density difference, and water viscosity.
2.Settling velocity determines whether particles will settle before water exits the tank.
3.Other factors like temperature, tank shape, and coagulant type influence performance, but the primary determinant is settling velocity
​
​Other Options:
​(ক)Water temperature: It affects water viscosity slightly, influencing settling, but not as significantly as settling velocity itself.
(খ)Shape of tank : Helps in proper flow distribution and reducing short-circuiting but does not change the inherent ability of particles to settle.
(গ)Type of coagulant : Coagulants improve floc size and density (thus indirectly affecting settling velocity), but the key parameter remains the velocity of the particles formed.ant type influence performance, but the primary determinant is settling velocity.

Full name: Aluminum sulfate  (Al₂(SO₄)₃⋅18H₂O)
Role: Neutralizes negative charges on colloidal particles so they can clump (flocculate) and settle.
Most widely used: Because it is cheap, effective, and works for a wide pH range (typically 5.5–7.5).
​ Reaction:
                  Al₂(SO₄)₃ + 6H₂O → 2Al(OH)₃↓ + 3H₂SO₄
Al(OH)₃ → gelatinous floc that traps impurities.
H₂SO₄ → lowers pH slightly, often corrected by lime addition.
​
​Other Options:
(ক)​ Ferric chloride : Effective coagulant but less commonly used in drinking water than alum (used more in industrial wastewater).
(খ) Sodium chloride : Just table salt—does not coagulate particles.
(গ) Bleaching powder: A disinfectant (chlorine source), not a coagulant.

1.Disinfection in wastewater treatment aims to kill or inactivate pathogenic microorganisms. Physical methods mainly include heat (boiling) and UV radiation because they directly destroy microorganisms or their DNA.
2.Reverse osmosis is primarily a filtration process for removing dissolved salts, organic molecules, and some microbes, but it is not considered a standard disinfection method because its goal is separation, not pathogen inactivation.
3.Other options like boiling and UV radiation are widely recognized as physical disinfection methods. Chlorination, though listed, is a chemical method but still widely used for disinfection.
​
Other Options :
(ক) Boiling:Physical method; kills microorganisms by heat → commonly used for small-scale disinfection (household level).
(খ) Chlorination:Chemical method; uses chlorine to kill microbes → widely used, but not a physical method. Still effective for disinfection.
(ঘ) UV radiation:Physical method; damages microbial DNA and prevents replication → commonly used in water and wastewater plants.

1. Activated carbon is a highly porous material with a huge internal surface area (500–1500 m²/g), which allows it to adsorb organic molecules and certain chemicals from water.
2. The primary mechanism is physical adsorption, where organic contaminants adhere to the carbon surface through van der Waals forces or hydrophobic interactions.
3. This process is highly effective for removing color, odor, taste, and trace organic pollutants like pesticides and VOCs.

Other Options:
(ক) Ion exchange: Ion exchange resins remove charged ions (like hardness-causing Ca²⁺ and Mg²⁺), not organics.
(খ) Chemical precipitation: Used for removing dissolved metals or phosphates by forming insoluble compounds, not organics.
(ঘ) Electrolysis: Breaks down substances using electric current, not the mechanism in activated carbon filtration.

1.Chlorination efficiency depends on how well chlorine can contact and inactivate pathogens in water.
2.Low turbidity means fewer suspended particles that could shield microorganisms from chlorine. This ensures maximum exposure of pathogens to chlorine, improving disinfection.
3.High organic content, ammonia, or high pH can consume chlorine or reduce its effectiveness:
Organic matter → reacts with chlorine, forming chlorine demand.
Ammonia → forms chloramines, which are weaker disinfectants.
High pH (>9) → reduces the fraction of active hypochlorous acid (HOCl), decreasing disinfection efficiency.

Other Options :
(ক) High organic content: Consumes chlorine and reduces disinfection efficiency.
(খ) Presence of ammonia: Forms chloramines; slower and less effective than free chlorine.
(ঘ) High pH (>9): Reduces the amount of HOCl, the most effective disinfectant form of chlorine.

Advanced Oxidation Processes (AOP) aim to produce hydroxyl radicals (•OH), which are extremely powerful oxidants that attack and degrade organic pollutants.

Hydrogen peroxide (H₂O₂) is widely used because it can decompose under UV light or react with catalysts (e.g., Fe²⁺ in the Fenton process) to form hydroxyl radicals.

Other options like ozone or chlorine can oxidize pollutants but are less efficient in generating •OH radicals compared to H₂O₂-based systems.

So, H₂O₂ is the primary oxidant used to generate hydroxyl radicals in AOPs.

1.Chlorine demand is the amount of chlorine consumed by reactions with impurities in water before a residual remains.
​2.Initial chlorine dose formula:
 Initial dose=Chlorine demand+Desired free chlorine residual
            
3.Substituting the values:
 Initial dose=1.6 mg/L+0.4 mg/L=2.0 mg/
​
Other Options Analysis:
(ক) 1.6 mg/L: Only meets the chlorine demand; no residual will remain.
(খ) 1.8 mg/L: Partial residual; free chlorine < 0.4 mg/L.
(ঘ) 2.2 mg/L: Slightly more than needed; free chlorine residual would exceed target.

Phosphate treatment in boiler feed water is primarily used to prevent scale formation caused by hardness ions like calcium (Ca²⁺) and magnesium (Mg²⁺). The phosphate reacts with these ions to form soft sludge (mainly calcium phosphate), which can be easily removed, rather than forming hard scale on boiler tubes.

​Other Options:
খ) To control corrosion – This is mainly done by adding oxygen scavengers like sodium sulfite or neutralizing amines, not phosphates.
গ) To neutralize silica – Silica removal is handled by other methods, such as adding sodium hydroxide or using demineralization.
ঘ) To prevent foaming – Antifoaming agents or proper blowdown control are used for this purpose, not phosphate.

Low pH (acidic) water directly attacks and dissolves the protective iron oxide layer on the inside of the boiler and feedwater pipes. This exposes the raw steel to further corrosion, leading to rapid thinning, pitting, and ultimately, equipment failure. The other options are consequences of different issues:

a) Scaling is caused by hardness minerals (calcium, magnesium) and is a problem of high pH, not low pH.
c) Foaming is caused by high dissolved solids or contaminants, not directly by low pH.
d) Steam temperature is primarily a function of boiler pressure, not feedwater pH.

High silica content in boiler feed water can cause foaming and priming:
Foaming: Formation of bubbles on the water surface inside the boiler due to dissolved silica and other impurities.
Priming: Carryover of water along with steam, which can damage turbines and steam-using equipment.

Other options:
ক) Scale formation on tubes – Mostly caused by hardness (Ca²⁺, Mg²⁺), not silica.
খ) Increased oxygen corrosion – Caused by dissolved oxygen, not silica.
ঘ) Reduced alkalinity – Alkalinity is controlled chemically, silica doesn’t directly reduce it.

In electrodialysis, ions migrate through ion-exchange membranes under the influence of an electric field. The selectivity of this migration depends mainly on:
1.Ion charge: Higher charged ions (e.g., divalent Ca²⁺) migrate differently than monovalent ions (e.g., Na⁺).
2.Ion size: Smaller ions pass through membranes more easily than larger ions.

Other Options:
খ)Membrane pore size – Not the primary factor; electrodialysis relies on ion-exchange properties, not physical sieving.
গ)Operating pressure – Not significant; pressure-driven processes are for reverse osmosis, not electrodialysis.
ঘ)Flow velocity – Affects mass transfer rate but not inherent selectivity.

Ultrafiltration (UF) membranes are prone to biofouling, which is the accumulation of microorganisms, bacteria, and their extracellular products on the membrane surface. This limits long-term performance by:
    1.Reducing permeate flow.
    2.Increasing transmembrane pressure.
    3.Shortening membrane lifespan.

Other options:
ক) Scaling – Mainly caused by precipitation of hardness salts; important but usually controlled by pre-treatment.
গ) High pressure drop – A consequence, not the primary cause.
ঘ) Osmotic backflow – Not relevant for ultrafiltration; more a concern in reverse osmosis.

Activated carbon is a highly porous material with a large surface area, making it very effective for adsorption of dissolved organic compounds, chlorine, and substances that cause bad taste and odor in water. It does not:

Reduce hardness (that requires lime-soda or ion exchange)

Kill pathogens (that’s the role of disinfectants like chlorine or ozone)

Precipitate salts (done by chemical softening or reverse osmosis)

In a hybrid ultrafiltration (UF) + activated carbon (AC) system:
1.Ultrafiltration acts as a pre-treatment step, removing suspended solids, colloids, and turbidity.
2.This prevents clogging or fouling of the downstream activated carbon bed, ensuring effective adsorption of organic contaminants.

Other options:
ক) To disinfect water first – UF may remove some microorganisms, but disinfection is not the main reason.
খ) To lower pH for better adsorption – pH adjustment is independent of UF placement.
ঘ) To reduce organic content – AC primarily removes organics; UF mainly removes particulates.

1.BOD (Biochemical Oxygen Demand) measures the amount of oxygen required by microorganisms to degrade biodegradable organics.
2.COD (Chemical Oxygen Demand) measures the total oxygen required to chemically oxidize all organic matter, including both biodegradable and non-biodegradable fractions.
If BOD ≪ COD, it means:
    1.Most of the organic matter cannot be biologically degraded (non-biodegradable or slowly degradable).
    2.Only a small fraction contributes to microbial oxygen demand.

Other options:
ক) Water is rich in biodegradable organics – This would make BOD close to COD.
গ) Presence of high dissolved oxygen – Does not affect the BOD/COD ratio directly.
ঘ) The sample is well aerated – Aeration affects dissolved oxygen but not the intrinsic biodegradability ratio.